When an user or an operator of a recording device, such as a magnetic tape recorder, intends to monitor the information prerecorded on a recording medium, such as a magnetic tape, it is necessary to reproduce the audio signals at a normal playback speed. However, it takes a long period of time to search a given piece of information if a magnetic tape is played back from beginning to end at the normal playback speed. Therefore, according to a conventional technique the magnetic tape is reeled at a speed much higher than the normal playback speed, while the information prerecorded on the magnetic tape is reproduced via a reproduce head. Since the fast forward reeling speed is usually between 10 and 100 times the normal playback speed, it is almost impossible to ascertain the reproduced audio signals in detail, while it may be possible to detect whether the reproduced audio signal is a vocal sound or a piece of music; or whether the reproduce head is scanning a recorded portion or a blank portion (a portion on a magnetic tape on which no signal has been prerecorded). Accordingly, in order to obtain the information prerecorded on a magnetic tape within a relatively short period of time, the operator of a tape player has to repeat maniputation of switches of the tape player so that the tape is reeled at a fast forward speed and a normal playback speed cyclically. Searching a piece of prerecorded information in this method is troublesome and inconvenient.
In order to remove the above mentioned inconvenience apparatus for monitoring prerecorded information during fast reeling of a magnetic tape was devised in the past. This conventional apparatus employs a charge-transfer-device (CTD), and thus expands the time axis of the prerecorded information reproduced at a speed higher than the recording speed by changing the frequency of drive pulses applied to the charge-transfer-device so that the time axis corresponds to the original recording speed. Namely, if the information prerecorded on a magnetic tape is a vocal sound, the above mentioned apparatus samples each phoneme which constitute a vocal sound for very short period of time. As a result, when listening to the reproduced sound, the time axis of which has been expanded, it is extremely difficult to understand the contents of the reproduced audio sounds if the audio signals are reproduced from the magnetic tape at a speed much higher than the normal playback speed. In detail, the above mentioned apparatus effectively operates only when he magnetic tape is reeled at a speed less than twice or three times the normal playback speed. Therefore, when the magnetic tape is reeled at a speed between 10 and 100 times the normal playback speed, it is impossible to understand the contents of the vocal sounds. Furthermore, if the recorded information is a piece of music, the music will be reproduced in a distorted form, while the musical notes might be deviated from the original notes. This conventional apparatus was disclosed in a magazine, POPULAR SCIENCE, issued on January 1975 in the name of VSC (variable speech control) tape recorder. A similar technique was also disclosed in a U.S. Pat. No. 3,786,195 to Shiffman.
Accoding to the technique of VSC a tape is reeled at a speed twice faster than the recording speed so that the information prerecorded on the magnetic tape is originally reproduced at this fast speed. The information reproduced is repeatedly sampled, and each sampling period is about 20 milliseconds than which most phonemes (the sounds by which we recognize speech) are longer. The sampled pieces of information are successively stored in a bucket-brigade shift register, which is a kind of the above mentioned charge-transfer-device, and then read out in a sequence at a lower speed than they were stored so that the reproduced speech is stretched out to a length which equals twice the compressed period.
This conventional apparatus is useful when it is intended to effect fast listening along the entire tape. However, as mentioned hereinabove, when the magnetic tape is reeled at a speed over approximately three times the recording speed, the conventional apparatus sacrifices clarity or intelligibility. Consequently, the conventional apparatus is not suitable for searching a given piece of information prerecorded on a magnetic tape. Furthermore, it is impossible to add the above mentioned VSC system to a regular tape recorder since VSC requires a variable-speed-playback motor and motor control synchronized with the variable-delay line. In other words, according to the VSC technique a special circuit has to be built in a tape recorder.